When used correctly it means that the buyer has made an offer contingent on selling his/her property first. Which then means the seller has accepted the offer but has a "release" as a part of the contract.
Which means that if the seller receives another offer from a different buyer, they will request the first offer to either "release" the contingency and move forward with the purchase or "release" the contract so that the seller may contract with the new buyer. Normally there is a period given to the "release" (i.e. 48 hours).
However, I have seen agents use this incorrectly and apply it to any contingency. They will indicate PR until the contingency periods are released (i.e. physical, appraisal, loan, etc) and then move it to PN (pending no-show). This implies that the seller can release the contract during the contingency period and that is not really true.
PS means that there is a contract, but the seller would like to continue showing the property. This can happen when the seller does not have confidence the buyer can go through with the contract and want to keep collecting "back -up " offers.